If a space craft is to travel at 0.6 c, how must one design a protection system such that any and all debris collisions with the sail do not render it damaged for further use? Is it possible to employ a Whipple shield mechanism?
A lightsail capable of achieving any useful speed, let along 0.6c, must be both very large and very low mass. So typical speculative designs are vast sheets of very thin film, or even a mesh of tiny wires with holes smaller than the wavelength of the light used for propulsion. (see Discussion on wikipedia). In all cases I am aware of, the approach to collision shielding is the same -- design the sail to (a) tolerate having small holes (b) prevent such holes growing too large (for instance by enbedding regular reinforcing fibres in the sail (a little like "ripstop" nylon used for clothes and tents). It always seems to work out better to add more sail and tolerate some holes than add shielding to prevent holes. Of course a collision with the actual payload, rather than with the sail would be a huge problem, but also very unlikely.
At velocities like 0.6c, you also have to worry about interstellar hydrogen, as each atom now packs a significant amount of energy (about 150 MeV). Two approaches to dealing with this have been suggested to my knowledge: (1) use UV lasers to ionize the gas ahead of you, and then use a magnetic field to deflect the ions (2) choose a material for your sail that is resistant to erosion/weakening by such radiation (which would require a compromise with other considerations such as strength and reflectivity).
In order for a solar sail to work, they have to have an enormous surface area.
The amount of material needed to make such a thing would be huge let alone the additional materials needed to protect it from debris traveling faster than bullets.
Instead of trying to protect it from damage, it would be better to expect to get damaged and design a robust maintenance and repair process.
Currently, there are materials emerging with "self healing" properties. A variant of such materials could be used in the fabric of a solar sail such that it could autonomously or robotically self-heal the punctures.
There are currently a bunch of different materials with such traits that use different means of achieving the result. To my knowledge, none have yet been adapted for the rigors of space but that is not to say they couldn't be (insert money here). Here is one such example of a material that achieves this effect.
In the event a material can not be made to autonomously self heal in space perhaps it could be made to be robotically repaired. The sail could be lined with guide wires that could enable small robots to periodically scan and repair the sail surface.